Method of reestablishing in situ combustion in petroliferous formations



METHOD OF REESTABLISHING IN SITU COMBUS- TION IN PETROLIFEROUSFORMATIONS Clarke N. Simm and Coral L. De Priester, Fullerton, Califl,assignors to California Research Corporation, San Francisco, Calif., acorporation of Delaware "nited States Patent 2,793,697 Patented May 281957 No Drawing. Application July 5, 1055, Serial No. 520,163

7 Claims. (Cl. 166-39) 10 ods of reestablishing combustion were to floodthe burned-out zone with oil and reestablish combustion or to drill anew bore hole at substantially the interface between the burned-out zoneand the petroliferous-containing formation and reestablish combustion inthe petro- This invention relates in general to improvements in theproduction of gas and oil from subterranean petroliferous depositsthrough the use of in situ combustion, and relates more particularly toa method for reestablishing such combustion in a subterranean formation.

It is well known that conventional or primary methods of recovering gasand oil from subterranean formations, such as by pumping, often do notresult in recovery of more than 40% of the gas and oil present in theformation. Numerous secondary recovery methods, such as water floodingand gas drive by repressuring of the reservoir, are being practiced, buteven these methods have not resulted in as complete an economicalrecovery of the available gas and oil as desirable. Additionally,numerous methods have been proposed for improving recovery by thermalmeans involving in situ combustion of a portion of the deposit, eitherto lower the viscosity of the oil to facilitate production thereof or tocrack and distill the oil to obtain the more volatile portions thereof.In a copending application of Clarke N. Simm, Serial No. 379,729, thereis disclosed and claimed a method of secondary recovery utlizing in situcombustion in which the mass velocity of the free oxygen delivered tothe combustion zone is controlled within definite limits in accordancewith certain characteristics of the petroliferous formation to producestable and efficient operation of the combustion process.

In such methods of in situ combustion, a bore hole is drilled into thepetroliferous formation and a source of heat applied thereto to initiatecombustion in the portion of the petroliferous deposit adjacent the borehole. A suitable gaseous combustible mixture is then supplied down thebore hole to cause the combustion zone to progress out into theformation away from the bore hole and to produce cracking anddistillation of the more volatile components of the petroliferousdeposits and drive these components toward a production outlet oroutlets. The combustion process is supported primarily by combustion insitu of the residual or heavy ends of the petroliferous material presentin the formation. Ideally, once combustion is initiated in theformation, it is I maintained continuously until the desired recoveryoperations have been completed. However, in practice, the combustionzone may be extinguished through accident or through unavoidableconditions resulting from the nature of the formation itself. Forexample, such extinction may result from a prolonged breakdown of thecompressors supplying the gaseous mixture to move the combustion zone,or from a caving in of the bore hole from thermal stress which preventsfurther supply of the gaseous mixture to the combustion zone.

When such extinction occurs, the reelstablishing of the combustion zoneis very diflicult, since the zone between the bore hole and the point atwhich combustion was extinguished is substantially devoid ofpetroliferous material and therefore is incapable of supporting a secondcombustion zone passing therethrough. Furthermore, it is uneconomical toreestablish combustion by placing a liferous portion in the same mannerutilized to initially produce the combustion zone. However, thesemethods have the disadvantage of requiring considerable additional timeand expense in flooding the burned-out zone or drilling the additionalbore hole, particularly where the burned-out zone is quite extensive orthe formation being treated is located at a considerable depth below thesurface.

We have discovered that combustion may be reestablished in thepetroliferous formation by passing a hot front through the burned-outzone and carefully controlling the potential heat content of the gaseousmixture supplied to the hot front during its passage through theburned-out zone. In accordance with the present in vention, a hot frontis created in the original well bore by supplying a stoichiometricgaseous combustible mixture through the original well bore to theformation. This mixture is ignited and the supply is maintained untilapproximately the first foot of the formation out from the well bore isheated to a temperature greater than 1000 F. After establishment of thisminimum temperature in excess of 1000 F., the combustible gas content ofthe gaseous mixture is adjusted. so that substantially no combustion ofthe mixture occurs. That is, the combustible gas content of the mixtureis either reduced to a low value below that required to supportcombustion or is increased to a value sutficiently high to preventcombustion in the mixture. For example, at atmospheric pressure thelower limits of combustibility, expressed as percentage of total gas, ofrepresentative fuel gases are as follows: a typical natural gas, 4.5%;methane, 5.0%; ethane, 3.0%; propane, 2.12%. Thus, depending on theparticular type of fuel gas utilized, or combination of fuel gases, theconcentration thereof in the total mixture is maintained below thecorresponding lower limit of combustibility and this mixture is suppliedthrough the bore hole to cause the hot front to progress out through theformation. Alternatively, of course, the combustible gas content of themixture could be increased beyond the upper limit of combustibility toproduce substantially the same result.

As the hot front progresses out from the bore hole under the action ofthis flow of gas, the peak temperature of the hot front will decreasefrom radiation, conduction and convection losses to the surroundingformation and from heat losses to the gaseous mixture flowing throughthe formation. When the peak temperature of the hot front has decreasedto a predetermined minimum value, preferably no less than 1000 F. andabsolutely no less than 800 F., the combustible gas content of theinjected gaseous mixture is adjusted within the limits of combustibilityso that combustion may occur. Preferably, the combustible gas content isadjusted to in excess of 22% of equivalent methane, to supply additionalthermal energy to the hot front and to return the peak temperature ofthe hot front to a value greater than 1000 F. During this phase of theoperation, the hot front will remain substantially stationary in theformation while the peak temperature thereof increases from theincreased thermal energy being supplied from the combustion of thegaseous mixture.

When the'peak temperature of the hot front has increased to the desiredvalue, the combustible gas content of the gaseous mixture supplied tothe hot front is again adjusted so as to fall outside the limits ofcombustibility to cause the hot front to again move on out into theformation. When the peak temperature of the hot front again decreases tothe predetermined minimum value after progressing a certain distancethrough the burnedout zone, the combustible gas content of the gaseousmixture is again brought within the combustibility range to provideadditional thermal energy to raise the peak temperature of the hot frontto in excess of l000 F. The process is thus continued with alternatevariations in the combustibility of the gaseous mixture supplied to thehot front to cause the hot front to pass through the burned-out zonewith a minimum expenditure of thermal energy.

It will be noted that in our method, when the hot front is beingtransported through the formation, the concentration of combustible gasis outside the range of combustibility, and that when the hot front isrelatively stationary and the heat content thereof is being increased,the concentration of combustible gas is within the range ofcombustibility. It will be further noted in accordance with our methodthat when the lower limit of combustibility is utilized, the combustiblegas content of the gaseous mixture supplied to the hot front is at aminimum and that the oxygen content is at a maximum when the hot frontis being transported through the burned-out formation, and that thesituation is just the reverse when the hot front is relativelystationary and additional thermal energy is being supplied thereto bycombustion of the gas-rich mixture.

The method of the present invention was field tested under the followingconditions. In a test of in situ combustion for secondary recovery, acombustion zone was produced in an injection well by igniting phosphorustherein and supplying a gaseous combustible mixture down the injectionwell. After ignition of the phosphorus, a mixture comprising 15% fuelgas and 85% air was supplied at a rate of 20 cubic feet per minute. Thefuel gas contained 40% CO2, so that the effective fuel gas concentrationwas substantially 9%. Within six hours of the ignition of thephosphorus, the oxygen content of the effiuent gas from an observationwell located 20 feet from the injection well dropped to zero, indicatingcombustion of all the supplied oxygen and an excessive temperature atthe combustion zone. In corroboration of this, a thermocouple located inan adjacent well indicated a temperature of 2600 F. just before it fusedfrom the high temperature. The injection of fuel gas was stopped, andthe injection of air was continued for four days. During these fourdays, the oxygen content of the effluent gas increased from zero to 19%,indicating an absence of combustion in the formation. The injection ofair was then discontinued and the formation was left idle for ten days.

After these ten days, 320 pounds of phosphorus were injected into theinjection well and a stoichiometric mixture of fuel gas and air wasinjected at a rate of 20 cubic feet per minute. The phosphorus wasignited and the stoichiometric flow was continued until a hot fronthaving a temperature between 1200 F. and 1500 F. was produced in theformation adjacent the well bore. The fuel gas content of the mixturewas then decreased and the hot front was transported out into theformation by injection of a mixture comprising 6% fuel gas and 94% airat a rate of 50 C. F. M. The 6% fuel gas again contained 40% CO2 so thatthe effective fuel gas concentration was 3.6%. When the temperature ofthe hot front had decreased to approximately 1100 F., the etfective fuelgas content-of the injected mixture was 4 increased to 24% to providethermal energy to re-elevate the temperature of the hot front toapproximately l500 F. The fuel gas content of the injected mixture wasthen decreased again to 3.6% and the hot front transported an additionaldistance out into the formation.

This alternate injection of relatively lean and rich mixtures wascontinued until the hot front arrived at a position in the formationwhere combustible material was available. The arrival of the hot frontat a petroliferous-containing portion of the formation was indicated bya steady state low oxygen concentration in the effluent gas from theadjacent observation well, as contrasted to the rapid increase in oxygencontent of the effluent gas which occurred during injection of thegas-lean mixture when the hot front was being moved through theburnedout zone. This rapid increase in oxygen content, of course,resulted from the lack of any combustible material in the burned-outportions of the formation, and the low, steady state oxygenconcentration indicated the occurrence of combustion with its consequentoxygen consumption.

During the initial phases of the operation, cycling periods of eighthours were utilized, and the length of these cycling periods wasincreased to twenty-four hours in order to move ahead more rapidly whenit was verified that the thermal stability of the method was suflicientto permit these expanded periods of operation; that is, the temperatureof the front remained above 1100 F. for about the 24 hour period. Therate of injection of the mixtures was increased over a period of oneweek from the original 20 cubic feet per minute to cubic feet perminute. The total distance through which the hot front was transportedby the method of the present invention could not be ascertained exactly,but it was estimated, on the basis of thermal calculations, to bebetween two feet and ten feet. However, there is no reasonable limit onthe distance through which reignition could be established.

Although but a few illustrative embodiments of the present inventionhave been described, it will be apparent to those skilled in the artthat various changes and modifications may be made therein withoutdeparting from the spirit of the invention or the scope of the appendedclaims.

We claim:

1. The method of establishing combustion in a subterranean petroliferousdeposit which is separated from a well bore by a zone substantiallydevoid of petroliferous material comprising the steps of heating theportion of said zone adjacent said well bore to a temperature in excessof 1000" F. to produce a hot front, supplying a gaseous mixturecontaining a combustible gas through said well bore to said hot front,adjusting the concentration of said combustible gas in said mixture to avalue outside the range of combustibility of said mixture to transportsaid hot front into said zone until the temperature of said hot frontdecreases to a predetermined value, readjusting the concentration ofsaid combustible gas in said mixture to a value within the range ofcombustibility of said mixture to produce combustion for reelevating thetemperature of said hot zone to in excess of 1000" F., and alternatelyso varying the concentration of said combustible gas in said gaseousmixture to cause said hot front to progress through said zone to saidpetroliferous deposit.

2. The method of establishing combustion in a subterranean petroliferousdeposit which is separated from a well bore by a zone substantiallydevoid of petroliferous material comprising the steps of heating theportion of said zone adjacent said well bore to a temperature in excessof 1000 F. to produce a hot front, supplying a gaseous mixturecontaining a combustible gas through said well bore to said hot front,adjusting the concentration of said combustible gas in said mixture to avalue below the level of combustibility of said mixture to transportsaid hot front into said zone until the temperature of said hot frontdecreases to a predetermined value, increasing the concentration of saidcombustible gas in said mixture to a value within the range ofcombustibility of said mixture to produce combustion for reelevating thetemperature of said hot zone to in excess of 1000 F., and alternately sovarying the concentration of said combustible gas in said gaseousmixture to cause said hot front to progress through said zone to saidpetroliferous deposit.

3. The method of establishing combustion in a subterranean petroliferousdeposit which is separated from a well bore by a zone substantiallydevoid of petroliferous material comprising the steps of heating theportion of said zone adjacent said well bore to a temperature in excessof 1000 F. to produce a hot front, sup plying a gaseous mixturecontaining a combustible gas through said well bore to said hot front,adjusting the concentration of said combustible gas in said mixture to avalue above the level of combustibility of said mixture to transportsaid hot front into said zone until the temperature of said hot frontdecreases to a predetermined value, decreasing the concentration of saidcombustible gas in said mixture to a value within the range ofcombustibility of said mixture to produce combustion for reelevating thetemperature of said hot zone to in excess of 1000 F., and alternately sovarying the concentration of said combustible gas in said gaseousmixture to cause said hot front to progress through said zone to saidpetroliferous deposit.

4. The method of establishing combustion in a subterranean petroliferousdeposit which is separated from a well bore by a zone substantiallydevoid of petroliferous material comprising the steps of heating theportion of said zone adjacent said well bore to a temperature in excessof 1000 F. to produce a hot front, supplying a gaseous mixturecontaining methane through said well bore to said hot front to transportsaid hot front into said zone until the temperature of said hot frontdecreases to a predetermined value, the concentration of said methane insaid mixture not exceeding 5.0% to prevent combustion in said mixtureduring said transporting, increasing the concentration of said methanein said mixture to in excess of 22% to produce combustion in saidmixture to reelevate the temperature of said hot front to in excess of1000 F., and alternately so varying the concentration of said methane insaid gaseous mixture to cause said hot front to progress through saidzone to said petroliferous deposit.

5. The method of establishing combustion in a subterranean petroliferousdeposit which is separated from a well bore by a zone substantiallydevoid of petroliferous material comprising the steps of heating theportion of said zone adjacent said Well bore to a temperature in excessof 1000 F. to produce a hot front, supplying a gaseous mixturecontaining ethane through said well bore to said hot front to transportsaid hot front into said zone until the temperature of said hot frontdecreases to a predetermined value, the concentration of said ethane insaid mixture not exceeding 3.0% to prevent combustion in said mixtureduring said transporting, increasing the concentration of said ethane insaid mixture to in excess of 22% to produce combustion in said mixtureto reelevate the temperature of said hot front to in excess of 1000 F.,and alternately so varying the concentration of said ethane in saidgaseous mixture to cause said hot front to progress through said zone tosaid petroliferous deposit.

6. The method of establishing combustion in a subterranean petroliferousdeposit which is separated from a well bore by a zone substantiallydevoid of petroliferous material comprising the steps of heating theportion of said zone adjacent said well bore to a temperature in excessof 1000 F. to produce a hot front, supplying a gaseous mixturecontaining propane through said well bore to said hot front to transportsaid hot front into said zone until the temperature of said hot frontdecreases to a predetermined value, the concentration of said propane insaid mixture not exceeding 2.12% to prevent combustion in said mixtureduring said transporting, increasing the concentration of said propanein said mixture to in excess of 22% to produce combustion in saidmixture to reelevate the temperature of said hot front to in excess of1000 F., and alternately so varying the concentration of said propane insaid gaseous mixture to cause said hot front to progress through saidzone to said petroliferous deposit.

7. The method of establishing combustion in a subterranean petroliferousdeposit which is separated laterally from a well bore by a zonesubstantially devoid of petroliferous material comprising the steps ofheating the portion of said zone adjacent said well. bore to atemperature in excess of 1000 F. to produce a hot front, supplying agaseous mixture containing a combustible gas through said well bore tosaid hot front to transport said hot front into said zone until thetemperature of said hot front decreases to a predetermined value below1000 F., the percentage of said combustible gas in said mixture duringsaid transporting not exceeding 3.5%, increasing the concentration ofsaid combustible gas in said mixture to in excess of 22% to producecombustion of said mixture to reelevate the temperature of said hotfront to approximately 15 00 F., and alternately so varying theconcentration of said combustible gas in said gaseous mixture to causesaid hot front to progress through said zone to said petroliferousdeposit.

References Cited in the file of this patent UNITED STATES PATENTS2,642,943 Smith June 23, 1953

1. THE METHOD OF ESTABLISHING COMBUSTION IN A SUBTERRANEAN PETROLIFEROUSDEPOSIT WHICH IS SEPARATED FROM A WELL BORE BY A ZONE SUBSTANTIALLYDEVOID OF PETROLIFEROUS MATERIAL COMPRISING THE STEPS OF HEATING THEPORTION OF SAID ZONE ADJACENT SAID WELL BORE TO A TEMPERATURE IN EXCESSOF 1000* F. TO PRODUCE A HOT FRONT, SUPPLYING A GASEOUS MIXTURECONTAINING A COMBUSTIBLE GAS THROUGH SAID WELL BORE TO SAID HOT FRONT,ADJUSTING THE CONCENTRATION OF SAID COMBUSTIBLE GAS IN SAID MIXTURE TOVALUE OUTSIDE THE RANGE OF COMBUSTIBILITY OF SAID MIX TURE TO TRANSPORTSAID HOT FRONT INTO SAID ZONE UNTIL THE TEMPERATURE OF SAID HOT FRONTDECREASES TO A PREDETERMINED VALUE, READJUSTING THE CONCENTRATION OFSAID COMBUSTIBLE GAS IN SAID MIXTURE TO A VALUE WITHIN THE RANGE OFCOMBUSTIBILITY OF SAID MIXTURE TO PRODUCE COMBUSTION FOR RELEVATING THETEMPERATURE OF SAID HOT ZONE TO IN EXCESS OF 1000* F., AND ALTERNATELYSO VARYING THE CONCENTRATION OF SAID COMBUSTIBLE GAS IN SAID GASEOUSMIXTURE TO CAUSE SAID HOT FRONT TO PROGRESS THROUGH SAID ZONE TO SAIDPETROLIFEROUS DEPOSIT.